System and Method for Automatic Runtime Position Sensor Offset Calibration in a Linear Motion System

ABSTRACT

A system automatically calibrates gains and/or offsets for each position feedback signal in order to reduce variations between position feedback signals for each sensor in a linear drive system. As a mover travels along a track segment, the segment controller records the position feedback signal output from each position sensor corresponding to a magnet on the mover passing the position sensor. The segment controller periodically monitors the position feedback values generated by one mover as it travels along the track segment and automatically updates the sensor gains as a function of a ratio of a target peak value to a measured peak value of the position feedback signal. The segment controller also records the position feedback signal from each sensor when no mover is traveling past the sensor. The segment controller periodically monitors the position feedback values received when no mover is present and automatically updates sensor offset values.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 16/201,464 filed Nov. 27, 2018, the entire contentsof which is incorporated herein by reference.

BACKGROUND INFORMATION

The present invention relates to motion control systems and, morespecifically, to automatically adjusting sensor gains for positionsensors used to detect the position of movers in a linear drive systemfor a motion control system, where the motion control systemincorporates multiple movers propelled along a track using the lineardrive system.

Motion control systems utilizing movers and linear drives can be used ina wide variety of processes (e.g. packaging, manufacturing, andmachining) and can provide an advantage over conventional conveyor beltsystems with enhanced flexibility, extremely high-speed movement, andmechanical simplicity. The motion control system includes a set ofindependently controlled “movers” each supported on a track for motionalong the track. The track is made up of a number of track segmentsthat, in turn, hold individually controllable electric coils. Successiveactivation of the coils establishes a moving electromagnetic field thatinteracts with the movers and causes the mover to travel along thetrack.

Each of the movers may be independently moved and positioned along thetrack in response to the moving electromagnetic field generated by thecoils. In a typical system, the track forms a closed path over whicheach mover repeatedly travels. At certain positions along the trackother actuators may interact with each mover. For example, the mover maybe stopped at a loading station at which a first actuator places aproduct on the mover. The mover may then be moved along a processsegment of the track where various other actuators may fill, machine,position, or otherwise interact with the product on the mover. The movermay be programmed to stop at various locations or to move at acontrolled speed past each of the other actuators. After the variousprocesses are performed, the mover may pass or stop at an unloadingstation at which the product is removed from the mover. The mover thencompletes a cycle along the closed path by returning to the loadingstation to receive another unit of the product.

A controller for the linear drive system requires position informationidentifying the location of each of the movers in order to activate theappropriate coil and control motion of each mover according to a desiredmotion profile. One method to provide the position information of eachmover to the controller is to place a position magnet on the mover andto provide a series of sensors spaced at fixed intervals along the trackthat detect the magnetic field generated by the position magnet. As amover travels along the track, different sensors detect the magneticfield of the position magnet and generate a position feedback signal forthe controller that is used to determine the location of each mover.This position feedback signal is an analog signal that varies inamplitude as a function of the relationship of the position magnet tothe sensor. The controller uses the amplitude of the position feedbacksignal to determine a location of the mover with respect to the positionsensor generating the feedback signal.

However, such a system for position sensing is not without certaindrawbacks. The controller uses the amplitude of a position feedbacksignal or of two adjacent position feedback signals to determine thelocation of a mover. Variation in the amplitude of the position feedbacksignal will result in variation in the detected position. The controllerexpects a feedback signal having a nominal waveshape corresponding to amagnet passing the sensor. Based on the nominal waveshape, thecontroller is able to precisely determine the distance that the magnetis from the sensor and, therefore, determine the location of the moveralong the track segment. It is known that position sensors, even of thesame style or model, will have some variation between sensors.Variations in the amplitude of the position feedback signal may also beintroduced due to manufacturing tolerances in the magnet (e.g.,different field strengths), sensor (e.g., electronic componenttolerances), or in assembly (e.g., positioning the magnet and sensor indifferent orientations or at different distances from each other). Theresulting variations in amplitude of feedback signals generated bymanufacturing and/or assembly tolerances cause the controller todetermine a position for the mover that varies from the actual positionof the mover as a function of these tolerances.

Because the controller utilizes position feedback information fromdifferent position sensors as the mover travels along the track,variations in the amplitude of the position feedback signal betweenadjacent sensors introduce some error in the position information forthe corresponding mover. These variations may appear either as stepchanges in position between two adjacent position sensors or create someripple on the position feedback signal as the mover travels along thetrack segment. Although the controller will compensate for thesevariations in the position feedback information, these step changes orthe ripple on the position feedback signal similarly result in stepchanges and/or ripple on the current generated by the coils used todrive the movers in an attempt to compensate for the variations inposition feedback information.

Thus, it would be desirable to provide a system to automaticallycalibrate gains and/or offsets for each position feedback signal inorder to reduce variations between position feedback signals from eachsensor.

BRIEF DESCRIPTION

The subject matter disclosed herein describes a system to automaticallycalibrate gains and/or offsets for each position feedback signal inorder to reduce variations between position feedback signals for eachsensor in a linear drive system. As a mover travels along a tracksegment, the segment controller records the position feedback signaloutput from each position sensor corresponding to a magnet on the moverpassing the position sensor. The segment controller determines peakvalues for each position feedback signal and compares the peak valuesagainst a target peak value. The segment controller then adjusts a gainvalue for each sensor by a ratio of the target peak value to a measuredpeak value. The segment controller periodically monitors the positionfeedback values generated by one mover as it travels along the tracksegment and automatically updates the sensor gains as previouslydescribed.

According to another aspect of the invention, the segment controller mayperiodically monitor the values of each position feedback signal duringan interval in which no mover is located proximate to a position sensor.During this interval, the position feedback signals should be zero. Thesegment controller may read the present value of each position feedbacksignal and automatically update the sensor offset value such that thefeedback signals from each position sensor are zero when no magnet froma mover is within a detection range for the sensor.

According to one embodiment of the invention, a system for automaticsensor offset determination in a linear drive system is disclosed. Thesystem includes a track defining a path along which multiple moverstravel and multiple position sensors spaced along the track. Each of theposition sensors generates a feedback signal responsive to at least oneof the plurality of movers traveling past the position sensor. Thesystem also includes a memory device and a processor in communicationwith the memory device. The memory device is operative to store thefeedback signal from each of the position sensors and to store aplurality of sensor offset values. Each sensor offset value correspondsto one of the position sensors. The processor receives the feedbacksignal from each of the position sensors and is operative to store thefeedback signal from each of the position sensors in the memory devicewhen no mover is traveling past the position sensor, to generate a newsensor offset value for each of the position sensors as a function ofthe stored feedback signal from each of the position sensors and of apreviously stored sensor offset value for each of the plurality ofposition sensors, and to overwrite the sensor offset value previouslystored in the memory device with the new sensor offset value for each ofthe of position sensors.

According to another embodiment of the invention, a method for automaticsensor offset calibration in a linear drive system is disclosed. Afeedback signal is received from each of multiple position sensors at aprocessor in a controller of the linear drive system. The positionsensors are spaced along a track defining a path along which multiplemovers in the linear drive system travel. The feedback signal isgenerated when no mover is traveling past each of the position sensors.The feedback signal received from each of the position sensors is storedin a memory device in the controller. The processor generates a newsensor offset value for each of the position sensors as a function ofthe feedback signal received and a previously stored sensor offset valuefor each of the position sensors. The previously stored sensor offsetgain value stored in the memory device is overwritten with the newsensor offset value for each of the position sensors.

These and other advantages and features of the invention will becomeapparent to those skilled in the art from the detailed description andthe accompanying drawings. It should be understood, however, that thedetailed description and accompanying drawings, while indicatingpreferred embodiments of the present invention, are given by way ofillustration and not of limitation. Many changes and modifications maybe made within the scope of the present invention without departing fromthe spirit thereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments of the subject matter disclosed herein areillustrated in the accompanying drawings in which like referencenumerals represent like parts throughout, and in which:

FIG. 1 is a schematic representation of an exemplary control system fora linear drive system according to one embodiment of the invention;

FIG. 2 is a sectional view of one embodiment of a mover and tracksegment included in the linear drive system taken at 2-2 of FIG. 1;

FIG. 3 is a bottom plan view of the exemplary mover of FIG. 2;

FIG. 4 is a partial side cutaway view of the mover and track segment ofFIG. 2;

FIG. 5 is a sectional view of another embodiment of a mover and tracksegment included in the linear drive system taken at 2-2 of FIG. 1;

FIG. 6 is a partial side cutaway view of the mover and track segment ofFIG. 5;

FIG. 7 is a partial top cutaway view of the mover and track segment ofFIG. 2;

FIG. 8 is a block diagram representation of the exemplary control systemof FIG. 1;

FIG. 9 is a graphical representation of a nominal position feedbacksignal compared to a non-ideal position feedback signal;

FIG. 10 is a graphical representation of multiple position sensorfeedback signals without calibration;

FIG. 11 is a graphical representation of multiple position sensorfeedback signals with calibration performed according to one embodimentof the invention;

FIG. 12 is a graphical representation of position error for one moveralong a segment of a track without calibration; and

FIG. 13 is a graphical representation of position error for one moveralong the segment of a track of FIG. 12 with calibration performedaccording to one embodiment of the invention.

In describing the various embodiments of the invention which areillustrated in the drawings, specific terminology will be resorted tofor the sake of clarity. However, it is not intended that the inventionbe limited to the specific terms so selected and it is understood thateach specific term includes all technical equivalents which operate in asimilar manner to accomplish a similar purpose. For example, the word“connected,” “attached,” or terms similar thereto are often used. Theyare not limited to direct connection but include connection throughother elements where such connection is recognized as being equivalentby those skilled in the art.

DETAILED DESCRIPTION

The various features and advantageous details of the subject matterdisclosed herein are explained more fully with reference to thenon-limiting embodiments described in detail in the followingdescription.

Turning initially to FIGS. 1-4, an exemplary transport system for movingarticles or products includes a track 10 made up of multiple segments12. According to the illustrated embodiment, multiple segments 12 arejoined end-to-end to define the overall track configuration. Theillustrated segments 12 are both straight segments having generally thesame length. It is understood that track segments of various sizes,lengths, and shapes may be connected together to form the track 10without deviating from the scope of the invention. Track segments 12 maybe joined to form a generally closed loop supporting a set of movers 100movable along the track 10. The track 10 is illustrated in a horizontalplane. For convenience, the horizontal orientation of the track 10 shownin FIG. 1 will be discussed herein. Terms such as upper, lower, inner,and outer will be used with respect to the illustrated trackorientation. These terms are relational with respect to the illustratedtrack and are not intended to be limiting. It is understood that thetrack may be installed in different orientations, such as sloped orvertical, and include different shaped segments including, but notlimited to, straight segments, inward bends, outward bends, up slopes,down slopes and various combinations thereof. The width of the track 10may be greater in either the horizontal or vertical direction accordingto application requirements. The movers 100 will travel along the trackand take various orientations according to the configuration of thetrack 10 and the relationships discussed herein may vary accordingly.

According to the illustrated embodiment, each track segment 12 includesan upper portion 17 and a lower portion 19. The upper portion 17 isconfigured to carry the movers 100 and the lower portion 19 isconfigured to house the control elements. As illustrated, the upperportion 17 includes a generally u-shaped channel 15 extendinglongitudinally along the upper portion 17 of each segment. The channel15 includes a bottom surface 16 and a pair of side walls 13, where eachside wall 13 includes a rail 14 extending along an upper edge of theside wall 13. The bottom surface 16, side walls 13, and rails 14 extendlongitudinally along the track segment 12 and define a guideway alongwhich the movers 100 travel. According to one embodiment, the surfacesof the channel 15 (i.e., the bottom surface 16, side walls 13 and rails14) are planar surfaces made of a low friction material along whichmovers 100 may slide. The contacting surfaces of the movers 100 may alsobe planar and made of a low friction material. It is contemplated thatthe surface may be, for example, nylon, Teflon®, aluminum, stainlesssteel and the like. Optionally, the hardness of the surfaces on thetrack segment 12 are greater than the contacting surface of the movers100 such that the contacting surfaces of the movers 100 wear faster thanthe surface of the track segment 12. It is further contemplated that thecontacting surfaces of the movers 100 may be removably mounted to thehousing 11 of the mover 100 such that they may be replaced if the wearexceeds a predefined amount. According to still other embodiments, themovers 100 may include low-friction rollers to engage the surfaces ofthe track segment 12. Optionally, the surfaces of the channel 15 mayinclude different cross-sectional forms with the mover 100 includingcomplementary sectional forms. Various other combinations of shapes andconstruction of the track segment 12 and mover 100 may be utilizedwithout deviating from the scope of the invention.

According to the illustrated embodiment, each mover 100 is configured toslide along the channel 15 as it is propelled by a linear drive system.The mover 100 includes a body 102 configured to fit within the channel15. The body 102 includes a lower surface 106, configured to engage thebottom surface 16 of the channel, and side surfaces 108 configured toengage the side walls 13 of the channel. The mover 100 further includesa shoulder 105 extending inward from each of the side surfaces 108. Theshoulder 105 has a width equal to or greater than the width of the rail14 protruding into the channel. A neck of the mover then extends upwardto a top surface 104 of the body 102. The neck extends for the thicknessof the rails such that the top surface 104 of the body 102 is generallyparallel with the upper surface of each rail 14. The mover 100 furtherincludes a platform 110 secured to the top surface 104 of the body 102.According to the illustrated embodiment, the platform 110 is generallysquare and the width of the platform 110 is greater than the widthbetween the rails 14. The lower surface of the platform 110, an outersurface of the neck, and an upper surface of the shoulder 105 define achannel 115 in which the rail 14 runs. The channel 115 serves as a guideto direct the mover 100 along the track. It is contemplated thatplatforms or attachments of various shapes may be secured to the topsurface 104 of the body 102. Further, various workpieces, clips,fixtures, and the like may be mounted on the top of each platform 110for engagement with a product to be carried along the track by the mover100. The platform 110 and any workpiece, clip, fixture, or otherattachment present on the platform may define, at least in part, a loadpresent on the mover 100.

The mover 100 is carried along the track 10 by a linear drive system.The linear drive system is incorporated in part on each mover 100 and inpart within each track segment 12. One or more drive magnets 120 aremounted to each mover 100. With reference to FIG. 3, the drive magnets120 are arranged in a block on the lower surface of each mover. Thedrive magnets 120 include positive magnet segments 122, having a northpole, N, facing outward from the mover and negative magnet segments 124,having a south pole, S, facing outward from the mover. According to theillustrated embodiment, two positive magnet segments 122 are located onthe outer sides of the set of magnets and two negative magnet segments124 are located between the two positive magnet segments 122.Optionally, the positive and negative motor segments may be placed in analternating configuration. In still other embodiments, a single negativemagnet segment 124 may be located between the positive magnet segments122. Various other configurations of the drive magnets 120 may beutilized without deviating from the scope of the invention.

The linear drive system further includes a series of coils 150 spacedalong the length of the track segment 12. With reference also to FIGS. 5and 7, the coils 150 may be positioned within a housing 11 for the tracksegment 12 and below the bottom surface 16 of the channel 15. The coils150 are energized sequentially according to the configuration of thedrive magnets 120 present on the movers 100. The sequential energizationof the coils 150 generates a moving electromagnetic field that interactswith the magnetic field of the drive magnets 120 to propel each mover100 along the track segment 12.

A segment controller 50 is provided within each track segment 12 tocontrol the linear drive system and to achieve the desired motion ofeach mover 100 along the track segment 12. Although illustrated in FIG.1 as blocks external to the track segments 12, the arrangement is tofacilitate illustration of interconnects between controllers. As shownin FIG. 2, it is contemplated that each segment controller 50 may bemounted in the lower portion 19 of the track segment 12. Each segmentcontroller 50 is in communication with a central controller 170 whichis, in turn, in communication with an industrial controller 180. Theindustrial controller may be, for example, a programmable logiccontroller (PLC) configured to control elements of a process linestationed along the track 10. The process line may be configured, forexample, to fill and label boxes, bottles, or other containers loadedonto or held by the movers 100 as they travel along the line. In otherembodiments, robotic assembly stations may perform various assemblyand/or machining tasks on workpieces carried along by the movers 100.The exemplary industrial controller 180 includes: a power supply 182with a power cable 184 connected, for example, to a utility powersupply; a communication module 186 connected by a network medium 160 tothe central controller 170; a processor module 188; an input module 190receiving input signals 192 from sensors or other devices along theprocess line; and an output module 192 transmitting control signals 193to controlled devices, actuators, and the like along the process line.The processor module 188 may identify when a mover 100 is required at aparticular location and may monitor sensors, such as proximity sensors,position switches, or the like to verify that the mover 100 is at adesired location. The processor module 188 transmits the desiredlocations of each mover 100 to a central controller 170 where thecentral controller 170 operates to generate commands for each segmentcontroller 50.

With reference also to FIG. 8, the central controller 170 includes aprocessor 174 and a memory device 172. It is contemplated that theprocessor 174 and memory device 172 may each be a single electronicdevice or formed from multiple devices. The processor 174 may be amicroprocessor. Optionally, the processor 174 and/or the memory device172 may be integrated on a field programmable gate array (FPGA) or anapplication specific integrated circuit (ASIC). The memory device 172may include volatile memory, non-volatile memory, or a combinationthereof. An optional user interface 176 may be provided for an operatorto configure the central controller 170 and to load or configure desiredmotion profiles for the movers 100 on the central controller 170.Optionally, the configuration may be performed via a remote deviceconnected via a network and a communication interface 178 to the centralcontroller 170. It is contemplated that the system controller 170 anduser interface 176 may be a single device, such as a laptop, notebook,tablet or other mobile computing device. Optionally, the user interface176 may include one or more separate devices such as a keyboard, mouse,display, touchscreen, interface port, removable storage medium or mediumreader and the like for receiving information from and displayinginformation to a user. Optionally, the system controller 170 and userinterface may be an industrial computer mounted within a control cabinetand configured to withstand harsh operating environments. It iscontemplated that still other combinations of computing devices andperipherals as would be understood in the art may be utilized orincorporated into the system controller 170 and user interface 176without deviating from the scope of the invention.

The central controller 170 includes one or more programs stored in thememory device 172 for execution by the processor 174. The systemcontroller 170 receives a desired position from the industrialcontroller 180 and determines one or more motion profiles for the movers100 to follow along the track 10. A program executing on the processor174 is in communication with each segment controller 50 on each tracksegment via a network medium 160. The system controller 170 may transfera desired motion profile to each segment controller 50. Optionally, thesystem controller 170 may be configured to transfer the information fromthe industrial controller 180 identifying one or more desired movers 100to be positioned at or moved along the track segment 12, and the segmentcontroller 50 may determine the appropriate motion profile for eachmover 100.

A position feedback system provides knowledge of the location of eachmover 100 along the length of the track segment 12 to the segmentcontroller 50. According to one embodiment of the invention, illustratedin FIGS. 2 and 4, the position feedback system includes one or moreposition magnets 140 mounted to the mover 100 and an array of sensors145 spaced along the side wall 13 of the track segment 12. The sensors145 are positioned such that each of the position magnets 140 isproximate to the sensor as the mover 100 passes each sensor 145. Thesensors 145 are a suitable magnetic field detector including, forexample, a Hall-Effect sensor, a magneto-diode, an anisotropicmagnetoresistive (AMR) device, a giant magnetoresistive (GMR) device, atunnel magnetoresistance (TMR) device, fluxgate sensor, or othermicroelectromechanical (MEMS) device configured to generate anelectrical signal corresponding to the presence of a magnetic field. Themagnetic field sensor 145 outputs a feedback signal provided to thesegment controller 50 for the corresponding track segment 12 on whichthe sensor 145 is mounted. The feedback signal may be an analog signalprovided to a feedback circuit 58 which, in turn, provides a signal tothe processor 52 corresponding to the magnet 140 passing the sensor 145.

According to another embodiment of the invention, illustrated in FIGS. 5and 6, the position feedback system utilizes the drive magnets 120 asposition magnets. Position sensors 145 are positioned along the tracksegment 12 at a location suitable to detect the magnetic field generatedby the drive magnets 120. According to the illustrated embodiment, theposition sensors 145 are located below the coils 150. Optionally, theposition sensors 145 may be interspersed with the coils 150 and located,for example, in the center of a coil or between adjacent coils.According to still another embodiment, the position sensors 145 may bepositioned within the upper portion 17 of the track segment 12 and nearthe bottom surface 16 of the channel 15 to be aligned with the drivemagnets 120 as each mover 100 travels along the tracks segment 12.

The segment controller 50 also includes a communication interface 56that receives communications from the central controller 170 and/or fromadjacent segment controllers 50. The communication interface 56 extractsdata from the message packets on the industrial network and passes thedata to a processor 52 executing in the segment controller 50. Theprocessor may be a microprocessor. Optionally, the processor 52 and/or amemory device 54 within the segment controller 50 may be integrated on afield programmable gate array (FPGA) or an application specificintegrated circuit (ASIC). It is contemplated that the processor 52 andmemory device 54 may each be a single electronic device or formed frommultiple devices. The memory device 54 may include volatile memory,non-volatile memory, or a combination thereof. The segment controller 50receives the motion profile or desired motion of the movers 100 andutilizes the motion commands to control movers 100 along the tracksegment 12 controlled by that segment controller 50.

Each segment controller 50 generates switching signals to generate adesired current and/or voltage at each coil 150 in the track segment 12to achieve the desired motion of the movers 100. The switching signals72 control operation of switching devices 74 for the segment controller50. According to the illustrated embodiment, the segment controller 50includes a dedicated gate driver module 70 which receives commandsignals from the processor 52, such as a desired voltage and/or currentto be generated in each coil 150, and generates the switching signals72. Optionally, the processor 52 may incorporate the functions of thegate driver module 70 and directly generate the switching signals 72.The switching devices 74 may be a solid-state device that is activatedby the switching signal, including, but not limited to, transistors,thyristors, or silicon-controlled rectifiers.

According to the illustrated embodiment, the track receives power from adistributed DC voltage. A DC bus 20 receives a DC voltage, V_(DC), froma DC supply and conducts the DC voltage to each track segment 12. Theillustrated DC bus 20 includes two voltage rails 22, 24 across which theDC voltage is present. The DC supply may include, for example, arectifier front end configured to receive a single or multi-phase ACvoltage at an input and to convert the AC voltage to the DC voltage. Itis contemplated that the rectifier section may be passive, including adiode bridge or, active, including, for example, transistors,thyristors, silicon-controlled rectifiers, or other controlledsolid-state devices. Although illustrated external to the track segment12, it is contemplated that the DC bus 20 would extend within the lowerportion 19 of the track segment. Each track segment 12 includesconnectors to which either the DC supply or another track segment may beconnected such that the DC bus 20 may extend for the length of the track10. Optionally, each track segment 12 may be configured to include arectifier section (not shown) and receive an AC voltage input. Therectifier section in each track segment 12 may convert the AC voltage toa DC voltage utilized by the corresponding track segment.

The DC voltage from the DC bus 20 is provided at the input terminals 21,23 to a power section for the segment controller. A first voltagepotential is present at the first input terminal 21 and a second voltagepotential is present at the second input terminal 23. The DC bus extendsinto the power section defining a positive rail 22 and a negative rail24 within the segment controller. The terms positive and negative areused for reference herein and are not meant to be limiting. It iscontemplated that the polarity of the DC voltage present between theinput terminals 21, 23 may be negative, such that the potential on thenegative rail 24 is greater than the potential on the positive rail 22.Each of the voltage rails 22, 24 are configured to conduct a DC voltagehaving a desired potential, according to application requirements.According to one embodiment of the invention, the positive rail 22 mayhave a DC voltage at a positive potential and the negative rail 24 mayhave a DC voltage at ground potential. Optionally, the positive rail 22may have a DC voltage at ground potential and the negative rail 24 mayhave a DC voltage at a negative potential According to still anotherembodiment of the invention, the positive rail 22 may have a first DCvoltage at a positive potential with respect to the ground potential andthe negative rail 24 may have a second DC voltage at a negativepotential with respect to the ground potential. The resulting DC voltagepotential between the two rails 22, 24 is the difference between thepotential present on the positive rail 22 and the negative rail 24.

It is further contemplated that the DC supply may include a thirdvoltage rail 26 having a third voltage potential. According to oneembodiment of the invention, the positive rail 22 has a positive voltagepotential with respect to ground, the negative rail 24 has a negativevoltage potential with respect to ground, and the third voltage rail 26is maintained at a ground potential. Optionally, the negative voltagerail 24 may be at a ground potential, the positive voltage rail 22 maybe at a first positive voltage potential with respect to ground, and thethird voltage rail 26 may be at a second positive voltage potential withrespect to ground, where the second positive voltage potential isapproximately one half the magnitude of the first positive voltagepotential. With such a split voltage DC bus, two of the switchingdevices 74 may be used in pairs to control operation of one coil 150 byalternately provide positive or negative voltages to one the coils 150.

The power section in each segment controller 50 may include multiplelegs, where each leg is connected in parallel between the positive rail22 and the negative rail 24. According to the illustrated embodiment,three legs are shown. However, the number of legs may vary and willcorrespond to the number of coils 150 extending along the track segment12. Each leg includes a first switching device 74 a and a secondswitching device 74 b connected in series between the positive rail 22and the negative rail 24 with a common connection 75 between the firstand second switching devices 74 a, 74 b. The first switching device 74 ain each leg 221 may also be referred to herein as an upper switch, andthe second switching device 74 b in each leg 221 may also be referred toherein as a lower switch. The terms upper and lower are relational onlywith respect to the schematic representation and are not intended todenote any particular physical relationship between the first and secondswitching devices 74 a, 74 b. The switching devices 74 include, forexample, power semiconductor devices such as transistors, thyristors,and silicon controlled rectifiers, which receive the switching signals72 to turn on and/or off. Each of switching devices may further includea diode connected in a reverse parallel manner between the commonconnection 75 and either the positive or negative rail 22, 24.

The processor 52 also receives feedback signals from sensors providingan indication of the operating conditions within the power segment or ofthe operating conditions of a coil 150 connected to the power segment.According to the illustrated embodiment, the power segment includes avoltage sensor 62 and a current sensor 60 at the input of the powersegment. The voltage sensor 62 generates a voltage feedback signal andthe current sensor 60 generates a current feedback signal, where eachfeedback signal corresponds to the operating conditions on the positiverail 22. The segment controller 50 also receives feedback signalscorresponding to the operation of coils 150 connected to the powersegment. A voltage sensor 153 and a current sensor 151 are connected inseries with the coils 150 at each output of the power section. Thevoltage sensor 153 generates a voltage feedback signal and the currentsensor 151 generates a current feedback signal, where each feedbacksignal corresponds to the operating condition of the corresponding coil150. The processor 52 executes a program stored on the memory device 54to regulate the current and/or voltage supplied to each coil and theprocessor 52 and/or gate driver module 70 generates switching signals 72which selectively enable/disable each of the switching devices 74 toachieve the desired current and/or voltage in each coil 150. Theenergized coils 150 create an electromagnetic field that interacts withthe drive magnets 120 on each mover 100 to control motion of the movers100 along the track segment 12.

As previously discussed, the position feedback system provides knowledgeof the location of each mover 100 along the length of the track segment12 to the corresponding segment controller 50. A magnetic fielddetector, such as a Hall-Effect sensor, generates a waveform that varieswith respect to the position of the mover 100 in relation to the sensor145 responsive to the position magnet 140 passing the sensor 145. Anominal position feedback signal 250 generated by a sensor 145 as theposition magnet 140 passes is illustrated in FIG. 9. The nominalposition feedback signal 250 corresponds to an ideal feedback signalgenerated by one of the position sensors 145 if the position magnet 140and position sensor 145 are each manufactured and installed according totheir respective nominal configuration and without variations inmanufacturing tolerances. The peak values of the nominal positionfeedback signal 250 correspond to a target peak value, the offset of thenominal position feedback signal is zero, such that the value of thenominal position feedback signal is at zero volts when no positionmagnet 140 from a mover 100 is within range of the sensor.

FIG. 9 also illustrates a second position feedback signal, where thesecond illustrated signal is a non-ideal position feedback signal 255for comparison to the nominal position feedback signal 250. As theillustrated non-ideal position feedback signal 255 is compared to thenominal position feedback signal 250, there are clear differencesbetween the signals. For example, at the zero position along thehorizontal axis, corresponding to time at which the position feedbacksignal is being generated with no position magnet proximate to theposition sensor 145, the nominal position feedback signal 250 is equalto zero. However, the non-ideal position feedback signal 255 is equal toa negative two-tenths, indicating an offset error in the non-idealposition feedback signal 255. Further, the difference between themaximum and minimum values of the nominal position feedback signal 250is approximately one and three-tenths. However, the difference betweenthe maximum and minimum values of the non-ideal position feedback signal255 is approximately eight-tenths, indicating a gain error in thenon-ideal position feedback signal 255. The variations in the feedbacksignals may be a result of manufacturing tolerances in the devices(e.g., the position magnet 140 or the sensor 145), in assembly of theposition magnet 140 on the mover 100, in assembly of the position sensor145 on the track segment 12, in the quality of the material covering theposition sensor and located between the sensor and the magnet, or acombination thereof. Variations in the feedback signals, however, cancreate variations in the determination of a position for a mover 100traveling along the track segment.

With reference next to FIG. 10, multiple position feedback signals 225are illustrated on a single graph 220. Each position feedback signal 225a-225 h corresponds to a position magnet 140 passing a position sensor145. The graph 220 is representative of two different scenarios. Thegraph 220 first may represent a single position magnet 140 passingmultiple position sensors 145. In this scenario, each position feedbacksignal 225 a-225 h corresponds to a different position sensor 145.Alternately, the graph 220 may represent multiple position magnets 140passing a single position sensor 145. In this scenario, each positionfeedback signal 225 a-225 h corresponds to a different position magnet140. The segment controller 50 utilizes one or more of the positionfeedback signals 225 in real-time to determine position information foreach mover 100 traveling along a track segment.

As a mover 100 travels along a track segment 12, the position feedbacksignals 225 may be stored in memory 54 on the segment controller 50 forfurther processing. As will be discussed in more detail below, thestored position feedback signals 225 may be used, for example, todetermine gains and/or offsets for each sensor in order to provide moreuniform feedback signals between different sensors. As a mover 100travels along a track segment 12, the segment controller 50 isconfigured to store position feedback signals 225 from each positionsensor 145 located along the track segment 12. If a mover 100 has asingle position magnet 140, a single set of feedback signals 225 for themagnet 140 is stored for each position sensor 145. If a mover 100includes an array of magnets 140, then a separate feedback signal 225may be stored for each position magnet 140 as it passes each sensor 145.In other words, if a mover 100 includes four position magnets 140 and atrack segment 12 includes eight sensors 145 spaced along its length,then thirty-two feedback signals 225 will be stored in the memory 54 foreach mover 100 as it passes along the track segment 12. According to oneembodiment of the invention, the memory 54 may include a table withmemory allocated for each mover 100 and each position magnet 140 locatedon the mover 100. According to another embodiment of the invention, thememory 54 may include a table with memory allocated for a single mover100. The processor 52 may be configured to store an identifier of eachmover as it travels along the track segment 12 and associate theidentifier with the set of feedback signals 225. Thus, furtherevaluation of the feedback signals 225 may identify a particular mover100 with which a particular set of feedback signals 225 may beassociated.

In order to account for variations in the feedback signals, it iscontemplated that a compensation table may be stored in the memory 54 ofthe segment controller 50. Initially, the nominal position feedbacksignal 250 may be utilized to generate the compensation table for eachof the position sensors 145 on the track segment 12. A position feedbacksignal 225 from each sensor 145 may be compared to the nominal feedbacksignal 250 which determines variations in the gain and/or offset presenton a particular feedback sensor 145. During a commissioning process, amover 100 having a position magnet 140 generating a known magnetic fieldmay be driven past each of the position feedback sensors 145 on thetrack segment 12. Each of the position feedback signals 225 generated asthe known position magnet 140 passes one of the position feedbacksensors 145 is compared to the nominal position feedback signal 250. Adifference between the values at the zero location, when the magnet 140on the mover 100 is not close enough to the sensor 145 to generate aposition feedback signal 225, may be stored in the compensation tablefor each sensor 145 to provide an initial offset compensation for eachposition feedback sensor 145. A difference between the maximum andminimum values may be stored in the compensation table for each sensor145 or, optionally, the processor 52 may use the difference to determinean initial sensor gain for each sensor 145 and the initial difference orthe initial sensor gain may be stored in the compensation table toprovide gain compensation.

After the commissioning run and during normal operation, the processor52 uses the offset and gain compensation values to normalize a positionfeedback signal prior to using the position feedback signal to determinethe present location of each mover 100. The processor 52 may add theoffset compensation value to a feedback signal 225 or multiply afeedback signal 225 by the gain compensation value corresponding to eachsensor 145 to generate the compensated feedback signal for each positionsensor 145 that is initially normalized to the nominal feedback signal250. Even if an initial set of sensor gains and/or sensor offsets arestored in a compensation table, these values may change over time orduring operation due, for example, to normal wear of the linear drivesystem or due to variations in operating conditions such as temperature,electromagnetic interference, and the like. Thus, the present inventionautomatically compensates sensor gain and/or offset information duringoperation of the linear drive system.

In operation, a controller in the linear drive system periodicallymonitors the position feedback signals 225 and automatically determinesnew sensor gain and/or offset information as the movers 100 aretraveling along the track 10. According to one embodiment, the segmentcontroller 50 is the controller used to automatically adapt the sensorgains and/or offsets. Optionally, the stored position feedback signals225 may be periodically transmitted to the central controller 170 or theindustrial controller 180 where the sensor gains and/or offsets maybeperiodically adjusted. For ease of discussion, the embodiment of theinvention with the segment controller 50 performing the adjustments willbe discussed.

The segment controller 50 creates a record of each of the positionfeedback signals 225. As illustrated by the nominal position feedbacksignal 250 in FIG. 9, the cycle begins at the zero position, increasesslightly to a first peak value, drops to a minimum value, increasesagain to a second peak value, and returns to the zero position. Theillustrated waveform is exemplary only and may vary according to thetype and/or the polarity of the magnetic field sensor used to detect theposition magnet 140. According to a first embodiment, the segmentcontroller 50 records each position feedback signal during an entirecycle of the position magnet 140 passing by the position sensor 145.Optionally, the processor 52 or a separate logic circuit may beconfigured to provide peak detection and the processor 52 may recordjust the peak values of each position feedback signal 225.

In either embodiment, the processor 52 uses the peak value, or values,of the position feedback signal 225 to determine a new sensor gain valuefor each of the position feedback signals. It is contemplated that theprocessor 52 may select one of the peak values, such as the minimumvalue as illustrated in FIG. 9, which has the greatest amplitude.Optionally, the processor 52 may determine a difference between themaximum and minimum values of the feedback signal and determine apeak-to-peak value of the position feedback signal 225. According toeither embodiment, the peak or peak-to-peak value becomes a measuredpeak value. Using the target peak value, the measured peak value, andthe existing sensor gain value, the processor 52 determines a new sensorgain value as shown below in Eq. 1. After determining the new sensorgain value, the processor 52 stores the new sensor gain value in thememory device 54.

$\begin{matrix}{K_{new} = \frac{{peak}_{target} \cdot K_{existing}}{{peak}_{measured}}} & (1)\end{matrix}$

According to another aspect of the invention, it is contemplated thatthe processor 52 may utilize an average of measured peak values ratherthan a single measured peak value to determine new sensor gain values.The processor 52 may, for example, store one or more prior values of themeasured peak value for each position feedback signal. The prior valueor values may be averaged together with the new measured peak value foreach position feedback signal. The average peak value of the positionfeedback signal may replace the measured peak value in Eq. 1 above.

While the process described above may be used generally to determine anew sensor gain for each of the position sensors 145, the processor 52may be configured to execute the steps only in response to certainevents or at certain times to avoid excessive processing steps beingrequired. According to one aspect of the invention, the sensor gainvalues may be determined initially each time the power is cycled. Theinitial set of sensor gain values stored in the compensation table maybe read from the memory device and written to a working set of sensorgain values, such that the initial set of sensor gain values remain forreference and are not overwritten. As each new sensor gain value isdetermined, it is written to the location in the set of working sensorgain values corresponding to the position sensor 145 for which it isdetermined. At power-up, the processor 52 may use each of the positionfeedback signals 225 generated by the first mover to travel along thetrack 10 in order to generate an initial set of adjusted sensor gainvalues. Optionally, each of the movers 100 may include an identifier,where the processor 52 maintains an association of each identifier, thecorresponding mover, and its location along the track 10. One of themovers 100 may be selected as a reference mover and the positionfeedback signals 225 generated when the reference mover travels alongthe track are used to generate new sensor gain values.

According to another aspect of the invention, it is contemplated thatthe processor 52 adjusts the sensor gain values for each position sensor145 during operation of the linear drive system. Because variations inthe position feedback signals 225 may occur over time, the segmentcontroller 50 is configured to periodically capture a set of positionfeedback signals 225 for the position sensors 145 located along thattrack segment and determine new sensor gain values. Operatingconditions, such as temperature or other electronic devices generatingelectromagnetic interference may impact the waveform of the positionfeedback signal 225. Consequently, it may be desirable to configure thesegment controller 50 to determine new sensor gain values at a shorterperiodic interval such as every few minutes or tens of minutes. Otherconditions, such as normal wear of the equipment may also impact thewaveform of the position feedback signal 225. Such wear occurs moreslowly, however, and may require the segment controller 50 to determinenew sensor gain values over a longer periodic interval such as once perday or once per week. According to still another option, the controlledsystem may include a calibration run, which may be initiated by theindustrial controller 180, during which one or more of the movers 100traverse the length of the track 10 and a new set of sensor gain valuesare determined. The frequency at which the sensor gain values aredetermined and automatically updated are dependent, therefore, on theapplication requirements.

As discussed above, ideal sensors would generate position feedbacksignals identical in shape to each other and having the same shape asthe nominal position feedback signal 250 as a position magnet 140 passesthe sensor. As illustrated in FIG. 10, it is contemplated that theposition sensors 145 may be spaced apart along the length of the tracksegment 12 at intervals that correspond to one-quarter of the wavelengthof the nominal position feedback signal 250. Spacing the positionsensors 145 at intervals equal to one-quarter of the expected wavelengthof the signal allows the processor 52 to utilize two adjacent signals ina quadrature manner to determine the position of the mover 100. Theexact shape of the waveform is less relevant but rather the uniformityof feedback signals from adjacent position sensors has greater relevancefor accurately determining the position of the mover.

As illustrated, however, in FIG. 10, the position feedback signals 225a-225 h from different position sensors 145 will vary. The magnitude ofa position error signal 300 is illustrated first in FIG. 12 utilizingposition feedback signals where the sensor gain values are notcompensated. FIG. 11 illustrates a second set of position feedbacksignals 225 a-225 h taken after the sensor gain values have beencompensated. The waveforms illustrated in FIG. 11, demonstrate a greateruniformity than those waveforms illustrated in FIG. 10. The resultingmagnitude of position error in the determination of the position foreach mover 100 is reduced as illustrated by the second position errorsignal 300 shown in FIG. 13.

According to another aspect of the invention, it is contemplated thatthe segment controller 50 may similarly be configured to automaticallyadjust a sensor offset value for each of the position sensors 145. Incontrast to determining the sensor gain value, the processor 52 isconfigured to periodically read the position feedback signal 225 when noposition magnet 140 is within range of the position sensor 145, suchthat the position feedback signal 225 is within the zero position. Theprocessor 52 stores the measured value of the position feedback signal225 while in the zero position and determines a new offset value.According to one embodiment, the new offset value may be a differencebetween the measured value of the position feedback signal 225 and anexpected value of the position feedback signal at the zero position.Optionally, the processor 52 may be configured to store multiplemeasured values of the position feedback signal and determine an averageof the measured value. The new offset value is applied to the measuredfeedback signal to shift the signal to the expected level when noposition magnet is present within the measurement range of the positionsensor 145.

It should be understood that the invention is not limited in itsapplication to the details of construction and arrangements of thecomponents set forth herein. The invention is capable of otherembodiments and of being practiced or carried out in various ways.Variations and modifications of the foregoing are within the scope ofthe present invention. It also being understood that the inventiondisclosed and defined herein extends to all alternative combinations oftwo or more of the individual features mentioned or evident from thetext and/or drawings. All of these different combinations constitutevarious alternative aspects of the present invention. The embodimentsdescribed herein explain the best modes known for practicing theinvention and will enable others skilled in the art to utilize theinvention.

We claim:
 1. A system for automatic sensor offset determination in alinear drive system, the system comprising: a track defining a pathalong which a plurality of movers travel; a plurality of positionsensors spaced along the track, wherein each of the plurality ofposition sensors generates a feedback signal responsive to at least oneof the plurality of movers traveling past the position sensor; a memorydevice operative to store the feedback signal from each of the pluralityof position sensors and to store a plurality of sensor offset values,wherein each sensor offset value corresponds to one of the plurality ofposition sensors; and a processor in communication with the memorydevice, wherein the processor receives the feedback signal from each ofthe plurality of position sensors and the processor is operative to: (a)store the feedback signal from each of the plurality of position sensorsin the memory device when no mover is traveling past the positionsensor, (b) generate a new sensor offset value for each of the pluralityof position sensors as a function of the stored feedback signal fromeach of the plurality of position sensors and of a previously storedsensor offset value for each of the plurality of position sensors, and(c) overwrite the sensor offset value previously stored in the memorydevice with the new sensor offset value for each of the plurality ofposition sensors.
 2. The system of claim 1 wherein the track includes aplurality of track segments, each track segment including: a portion ofthe plurality of position sensors spaced along the track, and a segmentcontroller, wherein the segment controller includes the memory deviceand the processor for the track segment and wherein the segmentcontroller is operative to generate the new sensor offset values foreach of the portion of the plurality of position sensors on the tracksegment.
 3. The system of claim 1 wherein each of the plurality ofmovers includes at least one magnet and each of the plurality ofposition sensors is a Hall-effect sensor operative to detect the atleast one magnet as each of the plurality of movers travels past theHall-effect sensor.
 4. The system of claim 1 wherein the new sensoroffset is generated by determining a difference between a nominal valueof a feedback signal from one of the plurality of position sensors andthe stored feedback signal from each of the plurality of positionsensors when no mover is traveling past the position sensor and addingthe difference to the previously stored sensor offset value.
 5. Thesystem of claim 1 wherein: the processor is further operative to storeat least one prior value of the feedback signal from each of theplurality position sensors when no mover is traveling past the positionsensor in the memory device, determine an average offset value for eachof the plurality of position sensors from the at least one prior value,and the new sensor offset is the average offset value determined foreach of the plurality of position sensors.
 6. The system of claim 1wherein the memory device is further operative to store a plurality ofsensor gain values, wherein each sensor gain value corresponds to one ofthe plurality of position sensors.
 7. The system of claim 6 wherein theprocessor is further operative to: (a) read the feedback signal fromeach of the plurality of position sensors responsive to at least one ofthe plurality of movers traveling past the position sensor, (b)determine a measured peak value of the feedback signal read from each ofthe plurality of position sensors, (c) generate a new sensor gain valuefor each of the plurality of position sensors as a function of a targetpeak value, of the measured peak value, and of the sensor gain value foreach of the plurality of position sensors, and (d) overwrite the sensorgain value stored in the memory device with the new sensor gain valuefor each of the plurality of position sensors.
 8. The system of claim 7wherein: a first mover is selected from the plurality of movers, theprocessor is further operative to monitor a present position of thefirst mover as the first mover travels along the track, and steps(a)-(d) of claim 7 are executed when each of the plurality of positionsensors generates the feedback signal responsive to the first movertraveling past the position sensor.
 9. The system of claim 8 wherein:the processor is further operative to store the feedback signal from atleast one of the plurality of position sensors for each of the pluralityof movers traveling past the at least one of the plurality of positionsensors, the measured peak value of each feedback signal from the atleast one of the plurality of position sensors is compared to identifythe feedback signal having a largest value, and the first mover isselected as the mover corresponding to the feedback signal having thelargest value.
 10. A method for automatic sensor offset calibration in alinear drive system, the method comprising the steps of: (a) receiving afeedback signal from each of a plurality of position sensors at aprocessor in a controller of the linear drive system, wherein theplurality of position sensors are spaced along a track defining a pathalong which a plurality of movers in the linear drive system travel andwherein the feedback signal is generated when no mover is traveling pasteach of the plurality of position sensors; (b) storing the feedbacksignal received from each of the plurality of position sensors in amemory device in the controller; (b) generating with the processor a newsensor offset value for each of the plurality of position sensors as afunction of the feedback signal received and a previously stored sensoroffset value for each of the plurality of position sensors; and (c)overwriting the previously stored sensor offset gain value stored in thememory device with the new sensor offset value for each of the pluralityof position sensors.
 11. The method of claim 10 wherein: the trackincludes a plurality of track segments, each track segment includes aportion of the plurality of position sensors spaced along the track anda segment controller, the segment controller includes the memory deviceand the processor for the track segment, and the segment controller isoperative to generate the new sensor offset values for each of theportion of the plurality of position sensors on the track segment. 12.The method of claim 10 wherein each of the plurality of movers includesat least one magnet and each of the plurality of position sensors is aHall-effect sensor operative to detect the at least one magnet as eachof the plurality of movers travels past the Hall-effect sensor.
 13. Themethod of claim 10 wherein generating the new sensor offset valueincludes determining a difference between a nominal value of a feedbacksignal from one of the plurality of position sensors and the storedfeedback signal from each of the plurality of position sensors when nomover is traveling past the position sensor and adding the difference tothe previously stored sensor offset value.
 14. The method of claim 10further comprising the steps of: storing at least one prior value of thefeedback signal from each of the plurality position sensors when nomover is traveling past the position sensor in the memory device,determining an average offset value for each of the plurality ofposition sensors from the at least one prior value, and the new sensoroffset is the average offset value determined for each of the pluralityof position sensors.
 15. The method of claim 10 further comprising thestep of storing a plurality of sensor gain values in the memory device,wherein each sensor gain value corresponds to one of the plurality ofposition sensors.
 16. The method of claim 15 further comprising thesteps of: (a) reading the feedback signal from each of the plurality ofposition sensors responsive to at least one of the plurality of moverstraveling past the position sensor, (b) determining a measured peakvalue of the feedback signal read from each of the plurality of positionsensors, (c) generating a new sensor gain value for each of theplurality of position sensors as a function of a target peak value, ofthe measured peak value, and of the sensor gain value for each of theplurality of position sensors, and (d) overwriting the sensor gain valuestored in the memory device with the new sensor gain value for each ofthe plurality of position sensors.
 17. The method of claim 16 furthercomprising the steps of: selecting a first mover from the plurality ofmovers; monitoring a present position of the first mover with theprocessor as the first mover travels along the track; and steps (a)-(d)are executed when each of the plurality of position sensors generatesthe feedback signal responsive to the first mover traveling past theposition sensor.
 18. The method of claim 17 further comprising the stepsof: storing the feedback signal from at least one of the plurality ofposition sensors for each of the plurality of movers traveling past theat least one of the plurality of position sensors; and identifying thefeedback signal having a largest peak value, wherein the first mover isselected as the mover corresponding to the feedback signal having thelargest peak value.